This invention relates to an image display device including scan electrodes arranged in a matrix-like manner, and more particularly to a display device which may be suitably realized in the form of an FED display device, an organic electroluminescence (hereinafter referred to as "EL") display device or the like.
When an electric field set to be about 10.sup.9 (V/m) is applied to a surface of a metal material or that of a semiconductor material, a tunnel effect occurs to permit electrons to pass through a barrier, resulting in the electrons being discharged to a vacuum even at a normal temperature. Such a phenomenon is referred to as "field emission" and a cathode constructed so as to emit electrons based on such a principle is referred to as "field emission cathode" (hereinafter also referred to as "FEC").
Recent development of semiconductor processing techniques permits a field emission cathode of the surface emission type to be constructed of an array of field emission cathode elements having a size as small as microns, so that an image display device (FED display device) having such field emission cathodes incorporated therein is currently subject to research and development.
Also, an organic EL display device which is another display device is likewise subject to research and development. The organic EL display device includes a luminous layer made of an organic compound while being based on an EL phenomenon wherein application of an electric field to a phosphor leads to luminescence of the phosphor.
In each of the image display device and organic EL display device, an improvement in quality of an image displayed requires to realize increased gradation expression. In order to control luminance of the display depending on an input video signal to realize satisfactory gradation expression, a system is employed wherein a signal which is subject to pulse width modulation (PWM) depending on the input video signal is used as a drive signal. Thus, a luminous time of each picture cell or pixel is controlled depending on the input video signal, resulting In gradation expression being carried out.
In general, pulse width modulation is attained by carrying out A/D conversion of an input video signal to obtain a digital data on the input video signal and detecting coincidence between the digital data and a value counted by a counter. In practice, the A/D conversion is restricted to a level as small as about 6 bits or the number of steps of gradation as small as 64 due to restriction of the number of wirings and a frequency of a clock for the counter. This renders realization of the number of steps of gradation as large as 256 or more at 8 bits highly difficult. Thus, the PWM system imposes restrictions to gradation expression, to thereby prevent the display from being provided with increased quality.
Also, a pulse amplitude modulation (PAM) system is proposed for the same purpose. It is adapted to execute gradation expression by modulating a drive voltage such as a voltage between a gate and a cathode (gate/cathode voltage) in the FED display device or a voltage between an anode and a cathode (anode/cathode voltage) in the organic EL display device. Unfortunately, the PAM system fails to accurately control gradation due to a variation in anode current leading edge point voltage on anode current characteristics (variation for every image cell pixel) in the FED display device or organic EL display device, temperature characteristics of a drive circuit, power loss or the like, to thereby fail to ensure increased display quality.